As known, systems for harvesting energy which allow to convert mechanical energy into electrical energy and to store the electrical energy thus obtained are available today.
For example, the article by T. T. Toh et al., “Continuously rotating energy harvester with improved power density”, Power MEMS 2008 Proceedings, pages 221-224, Sendai (JP), describes a system for harvesting energy of the continuously rotating type, which comprises a rotor, a stator and a counterweight, which is constrained to the stator. The rotor and the stator are part of a dynamo, thus the rotor is integral with a rotating element, which is actuated, for example, by an external motor.
In use, the external motor keeps the rotating element rotating. In turn, the rotor rotates together with the rotating element, while the counterweight tends to keep the stator fixed by applying a corresponding torque, which is caused by the force of gravity. A relative motion of the rotor with respect to the stator occurs, with consequent generation of electricity in the dynamo. Such an electrical power is transferred to a load, such as a battery, for example.
More in particular, the electrical power transfer causes the passage of current in the rotor, which in turn causes the application of a driving torque on the stator. The stator assumes a position during the rotation of the rotor which is distanced by an angular distance θ, measured along the direction of rotation, from the position that the stator itself would assume if the rotor were not rotating, which is also referred to as the ‘rest position’. Because the angular distance θ is not zero, the gravitational torque acting on the stator balances the driving torque so that the stator tends to remain immobile.
In greater detail, the power transfer occurs only if the rotation speed of the rotating element does not exceed a critical angular speed, beyond which the gravitational torque can no longer contrast the driving torque and the stator starts rotating together with the rotor. In practice, the gravitational torque assumes a maximum value when the angular distance θ of the counterweight from the rest position is equal to 90°; driving torques higher than such maximum value imply angular distances θ greater than 90°, which correspond to progressively lower gravitational torque values. Thus, when the angular distance θ of the counterweight exceeds a critical distance θc equal to 90°, the system for harvesting energy enters into a condition of instability, in which the counterweight tends to rotate at the same angular speed as the rotor, and the power transfer is essentially zero, due to the cancellation of the relative motion between stator and rotor.
In order to optimize the electrical power transfer, a so-called maximum power point tracking (MPPT) circuit is present between the dynamo and the load.
The MPPT circuit adapts the input impedance of the load to the output impedance of the dynamo, in order to maximize power transfer. In practice, the MPPT circuit adapts the impedance of the load to the impedance of the rotor armature.
In greater detail, the MPPT circuit comprises a so-called switching circuit, which is controlled by using a pulse-width-modulated (PWM) signal, generated by the MPPT circuit itself. The impedance is adapted by varying the duty cycle of such a pulse-width-modulated signal.
In even greater detail, the switching circuit has an electrical input, which is connected to the terminals of the rotor armature. An input current and an input voltage, delivered by the dynamo, are thus present on the electrical input of the circuit. The MPPT varies the duty cycle of the pulse-width-modulated signal as a function of the input current and the input voltage, correspondingly modulating the transfer of electrical power from the dynamo to the load.
The MPPT circuit thus allows to maximize the transfer of energy to the load, however its operation implies that the system for harvesting energy may become unstable. Indeed, as previously mentioned, the application of a load to the dynamo implies a braking action, which is greater the higher the current circulating in the rotor armature. Therefore, when attempting to transfer the maximum electrical power available at the dynamo terminals, the MPPT circuit may cause a braking action capable of making the counterweight rotate by an angle greater than the critical distance θc. In such a case, the system for harvesting energy becomes unstable. In order to prevent such an occurrence, the rotating element must be rotated at a speed considerably lower than the critical angular speed, and it is necessary to prevent the rotating element from being affected by linear accelerations, i.e. from translating, because such linear accelerations can contribute to reaching instability conditions. In other words, constraints must be introduced into the use of the system for harvesting energy.
It is the object of the present invention to provide a system for harvesting energy which at least partially solves the drawbacks of the prior art.